This invention relates to a mold for molding prefoamed material of thermoplastic synthetic resin.
The inventors of this application has already proposed this kind of mold in the Japanese Patent Publication No. 53-11994 (Published in 1978). As shown in FIG. 1, the mold of the above Patent Publication comprises a male mold 6 and a female mold 8, in which the former includes two heating and cooling chambers 4, 5 provided with a plurality of steam passages 3 within an internal wall 1 contacting foamed material of synthetic resin introduced in a cavity 2, while the latter includes a heating and cooling chamber 7.
When using the above mold, polyolefin particles foamed previously in other place are filled in the mold. At the first stage of heating, a steam is guided as shown by arrow X, and flows into firstly the chamber 4 of the male mold 6, the steam passages 3, the cavity 2, again the steam passages 3 and finally the chamber 5. Under the next heating, the steam is passed through the chamber 7, 4, 5 from an arrow Y, the polyolefin foamed particles being melted, and finally a preferred shape being formed.
However, this kind of mold has the following disadvantages and inconveniences.
(1) Since the female mold 8 including the chamber 7 makes it possible to adjust the steam pressure and raise the temperature to a necessary degree, the temperature of the previously foamed particles contacting the female mold 8 is much higher than that of the foamed particles in the midst of the cavity, so that a superficial skin layer is formed beautifully. However, in case of the male mold 6, at the beginning of heating, the steam introduced into the cavity 2 from the passages 3 escapes outside the mold through a parting line 9 between the two molds, 6, 8. As a result, the steam pressure is decreased and the heating and melting are going on with the low temperature steam. For this reason, a desired superficial skin layer is not formed on the portion contacting the male mold 6. In addition, there often remain some traces of bead interfaces on a surface of the foamed product. This is undesirable in view of external appearance. Moreover, the strength of the bead interfaces is weaker than that of the portion of the superficial layer, consequently the strength of the foamed product as a final product being not uniform. Further, in case of crystalline thermoplastic resin like polyolefin, a superficial layer of the foamed material may be affected seriously by a slight difference of the molding temperature.
(2) When increasing the steam pressure for raising the temperature of the male mold 6 having the steam passages 3 in order to eliminate the aforementioned temperature difference, there is the danger that the foamed material adjacent the steam passages 3 is worsened because holes are made on the surface of said foamed material.
(3) It is necessary to make the heating time longer.
The present invention aims at removing the above disadvantages of the conventional mold for molding prefoamed material of thermoplastic synthetic resin.